You already know that you can auto-complete web site addresses in Firefox's location bar using keyboard shortcuts like Ctrl+Enter and Ctrl+Shift+Enter. (Oh, you didn't? Here's how to http://, www, .com or .org again.) The How-To Geek explains how to tweak Firefox's about:config settings to alter the behavior of those keystrokes to your liking. So if you visit more .co.uk's more than .com's, Ctrl+Enter can fill in .co.uk instead. Just change the browser.fixup.alternate.suffix value to your desired suffix.